The present invention relates to a method of allocating channel slots to waiting calls coming from a plurality of user stations for an integration of constant and variable bit rate (CBR and VBR) traffic into a communication channel divided into frames each of which includes N(F) time slots. It also relates to a corresponding allocation system. This invention may be for instance implemented in a satellite network including VSAT terminals.
Satellite networks, that offer individual users a direct access to the satellite transponder resources from a VSAT (Very Small Aperture Terminal), are expected to play an important role in the multimedia communications. Since exchanging messages between a user terminal and the network control center costs a lot in terms of delay, the resource allocation policy for integrating different traffic types represents a major aspect in designing a satellite network able to support multimedia traffic generating applications. Channel access and bandwith attribution depend, to a large extent, on the type of traffic to the transferred, and the variation in traffic characteristics is therefore an essential factor in the choice of the resource allocation policy.
For synchronous traffic, that corresponds to time-sensitive applications generating stream traffic for long durations, its periodic nature and its time sensitivity necessitate the allocation of a fixed channel resource for the duration of a call, in a connection-oriented mode of service. The attribution of a channel for the whole call is particularly essential in the presence of the large satellite propagation delay, in order to limit the jitter variations for such real-time traffic. The synchronous traffic can then be attributed a constant bit rate class of service, as it occupies the same bandwidth resource during the whole call duration.
A synchronous traffic, on the other hand, corresponds to these situations where for instance interactive terminals, occasionally generating short bursts, can be admitted in the system for the duration of a burst, on a burst-by-burst reservation basis, and said traffic can then be supported on said basis in a connection-less mode of service. The burst can be mapped to a non-real time variable bit rate class of service, the non-real time nature arising from the fact that the reservation delay for an arriving burst is large compared to terrestrial networks (moreover, the resulting jitter cannot be guaranteed, since it largely depends on the access protocol applied and the variations of the link loading conditions).
An efficient resource allocation policy is therefore necessary to accomodate these service classes and at the same time to satisfy the quality of service required by each traffic type. In the satellite context, several strategies have been proposed to facilitate the use of the same channel for transmitting different traffic types. Generally the satellite uplink channel is subdivided into TDMA (Time Division Multiple Access) frames of fixed length. These frames are in turn divided into a given number L of time slots allocated to the different traffic types, either in a fixed manner (fixed boundary strategy) or in a variable one (movable boundary strategy).
With the fixed boundary strategy, each traffic type has a same number of time slots permanently allocated to it, and hence encounters no competition from other types to share its resources. This strategy may be inefficient in case resources are not fully utilized and therefore wasted by a given traffic type without any opportunity to be used by other ones.
Another strategy partially overcomes this drawback by allowing a limited sharing of resources: the so-called movable boundary strategy, described for instance in the document xe2x80x9cFixed- and movable boundary channel-access schemes for integrated voice/data wireless networksxe2x80x9d, J. E Wieselthier and A. Ephremides, IEEE Transactions on Communications, vol. 43, no1, January 1995, pp.64-74, and according to which short data messages can be allocated extra channels if they are not used by the stream traffic, but are however preempted by the latter if necessary.
Such a strategy, found to achieve a reduction of queueing delay for short data messages compared to the fixed boundary strategy (and therefore to achieve a better channel utilization), is now more precisely explained.
In a conventional movable boundary strategy, each frame is subdivided into two compartments:a CBR sub-frame, and a non-real time variable bit rate sub-frame (called NRT-VBR, or VBR, sub-frame). Since CBR calls occupy the channel resources for longer periods compared to the data bursts durations, CBR resources available at low loading conditions can be allocated to bursty data (arriving CBR requests continue however to have a higher priority in their own sub-frame): the boundary separating the two sub-frames can move inside the CBR one to allocate to VBR traffic the available time slots, but moves back to its position as CBR traffic increases (i.e. a CBR time slot that was borrowed for VBR traffic reverts to its original status as a CBR time slot immediately when needed for this purpose). This has the advantage of reducing the VBR traffic average delay, whereas the CBR performance is left unaffected. In contrast, the CBR traffic is not authorized to use a VBR time slot from the VBR sub-frame, since this. slot would then become unavailable to VBR traffic for the entire duration of the call.
An object of the invention is to propose an improved method of allocating time slots allowing to enhance the quality of service offered to CBR connections.
To this end the invention relates to a method as described in the preamble of the description and wherein, under normal loading conditions for both traffic types, each frame is subdivided thanks to two frontiers into a constant bit rate (CBR) traffic sub-frame composed of N(C) slots, a variable bit rate (VBR) data traffic sub-frame composed of N(Vmin) slots and, between them, a sub-frame called common resource pool (CRP) and composed of the (N(F)xe2x88x92N(C)xe2x88x92N(Vmin)) remaining slots of the frame, said allocation method then comprising the following steps:
(a) as the VBR traffic loading increases over that of CBR traffic, the boundary between them moves inside the CRP and the CBR sub-frame in order to include unallocated CBR channels;
(b) as the CBR traffic goes up, at small VBR loads said boundary moves in the CRP towards the VBR sub-frame, said displacement being limited by the minimum number of resources N(Vmin) permanently reserved for VBR traffic;
(c) under high loading conditions for both traffic types, CBR traffic resources are limited to N(C), while the remaining resources N(V)=N(F)xe2x88x92N(C) in the frame are available to VBR traffic.
This method, evaluated in a satellite environment, is more convenient than the conventional movable boundary strategy to integrate CBR and VBR traffic at lower loads of the latter, since it allows in the common time slot part mutual resource sharing, by dynamically adapting the allocation decision to different traffic load variations. The advantage of this method, carried out at the beginning of each frame in order to adapt it to said network loading conditions, is that it reduces CBR traffic blocking probability and call set-up delay at low VBR loads, because it allows CBR traffic to take advantage of the available resources on the frame at the same time of guaranteeing a minimum for the VBR traffic. The VBR queuing delay is also reduced at low CBR loading conditions. The proposed scheme however converges to the conventional movable boundary strategy at high load of both traffic types.
Another object of the invention is to propose an allocation system for carrying out said method.
To this end, the invention relates to a system for allocating channel slots to waiting calls coming from a plurality of user stations for an integration of constant and variable bit rate (CBR and VBR) traffic into a communication channel divided into frames each of which includes N(F) time slots, characterized in that it comprises a first finite length CBR queue, provided for storing arriving CBR call connections, a second infinite queue, provided for storing arriving VBR messages, and a frame allocation controller, provided for monitoring the filling level of each of the two queues at the beginning of control periods respectively associated to said frames, each frame being, under normal loading conditions for both traffic types, subdivided thanks to two frontiers into a CBR traffic sub-frame composed of N(C) slots, a VBR traffic sub-frame composed of N(Vmin) slots and, between them, a sub-frame called common resource pool (CRP) and composed of the (N(F)xe2x88x92N(C)xe2x88x92N(Vmin)) remaining slots of the frame, and an allocation method being then carried out
(a) as the VBR traffic loading increases over that of CBR traffic, the boundary between them moves inside the CRP and the CBR sub-frame in order to include unallocated CBR channels;
(b) as the CBR traffic goes up, at small VBR loads said boundary moves in the CRP towards the VBR sub-frame, said displacement being limited by the minimum number of resources N(Vmin) permanently reserved for VBR traffic
(c) under high loading conditions for both traffic types, CBR traffic resources are limited to N(C), while the remaining resources N(V)=N(F)xe2x88x92N(C) in the frame are available to VBR traffic.
In this system, the step (b) is preferably organized according to the following steps
(b1) before channels are allocated for the waiting calls from the CRP, the length of the second queue is monitored:
(i) if said length is less than a given threshold, CBR calls are granted access in the CRP sub-frame;
(ii) if said length exceeds said thresold, CBR requests are denied access to the CRP sub-frame, and are made to wait for a release of resources either in the CBR sub-frame or in the CRP sub-frame only if said queue length goes below said threshold;
(b2) if the first queue is completely filled when a new CBR call request arrives, said request is blocked and erased from the system.
In an advantageous embodiment of said system, the value of said threshold dynamically varies as a function of the number of CRP resources allocated to CBR traffic, said threshold being reduced as said number increases.